Components of a rolling cone bit mechanical face seal system utilized to seal the bearing typically include (A) two hard material components typically metal having surfaces engaged and sliding with relation to each other, (B) an elastomeric static seal ring with the primary function of providing an energizing force to one of the hard material components such that the surfaces of the hard material components are engaged at some designed contact pressure, (C) a second static sealing elastomer component sometimes referred to as a backup ring residing outside of a first elastomer component and engaged with one of the hard material components. This second elastomer component having the primary function of stopping ingress of the drilling environment into the annular space between one of the hard material seal components and the base area of the bearing pin which forms a gland for the elastomer energizer. This second static sealing elastomer component sometimes referred to as a backup ring often is the first component in the mechanical face sealing system to fail. Failure is typically in the form of tearing and wear generally initiating in the area of the outside diameter of the backup ring and on the surface engaged with one of the hard material seal components.
A backup ring (BUR) in a mechanical face seal assembly serves one or more of the following purposes: contribute to the face load; protect the energizer or energizing mechanism; provide resisting torque to prevent stationary seal from rotating; and fill the gland area to reduce the effect of mud packing. In the prior art, a low Shore A hardness elastomeric compound was used to meet the design requirements. Field experience shows that this material can degrade and often suffers tear and loses its function.
The basic assembly of a roller cone bearing seal assembly using a backup ring 55 is described in U.S. Pat. Nos. 6,142,249 and 7,168,147 which is presented below for context for the improvements to the backup ring contemplated by the present invention.
The numeral 11 in FIG. 1 of the drawing designates an earth-boring bit having a threaded upper portion 13 for connection to a drill string member (not shown). A fluid passage 15 directs drilling fluid to a nozzle (not shown) that impinges drilling fluid or mud against the borehole bottom to flush cuttings to the surface of the earth.
A pressure-compensating lubrication system 17 is contained within each section of the body, there usually being three, which are welded together to form the composite body. The lubrication system is preferably similar to that shown in U.S. Pat. No. 4,727,942, to Galle.
In each section of the body, a lubricant passage 19 extends from each compensator 17 downwardly into intersection with another lubricant passage 21 in which a ball plug 23 is secured to the body by a plug weld 25. Lubricant passages 27 carry lubricant to a cylindrical journal bearing surface defined between a cylindrical insert 29 (interference fit in cutter 33) and a corresponding cylindrical surface on bearing shaft 30, which is cantilevered downwardly and inwardly from an outer and lower region of the body of the bit, commonly known as the shirttail. Ball plug 23 retains a series of ball bearings 31 that rotatably secure cutter 33 to bearing shaft 30. Dispersed in the cutter are a plurality of rows of earth-disintegrating cutting elements or teeth 35 that may be constructed of a sintered tungsten carbide secured by interference fit into mating holes in cutter 33. A seal assembly 37, including a secondary seal is disposed adjacent the base of bearing shaft 30 and seals lubricant within the bearing and debris out of the bearing.
FIGS. 2 and 3 are enlarged section views of the bearing and seal assembly of the earth-boring bit. A pair of axial surfaces 39, 41 formed in cutter 33 and last-machined surface 43 of the shirttail portion of the bit body cooperate with a pair of radial surfaces 45, 47 to define a bearing seal gland generally at the base of bearing shaft 30. A seal assembly 37 is disposed in the seal gland and includes a rigid seal ring 49 and an o-ring energizer 51, which urges a seal face 53 on ring 49 into sealing engagement with a corresponding seal face 41 on an insert 29 in cutter 33. This rigid face seal is formed in accordance with U.S. Pat. No. 4,753,304, to Kelly.
Seal assembly 37 may be regarded as a primary seal because it is designed to seal the journal bearing against entry of foreign material or debris and to accommodate pressure fluctuations in the lubricant. Seal 37 is also a dynamic seal because it seals the moving or dynamic interface between each cutter and its bearing shaft and the relative rotational movement between them.
In addition to dynamic seal 37, a secondary or backup seal ring 55 is disposed in the seal gland opposite between seal assembly 37 and last-machined surface 43 to seal the seal gland and seal assembly 37 against entry of debris, particularly drilling mud particles, from the exterior of bit 11. To accommodate seal ring 55 and seal 37, axial surface 39 is in a groove machined into last-machined surface 43 to a depth approximately one-third to one-half the nominal axial thickness of ring 55. Axial surface 39 may be flush with last-machined surface 47.
FIG. 4 is an enlarged cross-section view of ring 55. Preferably, secondary seal ring 55 is a continuous ring formed of nitrile elastomer material of about 40-45 durometer (Shore A) and a modulus of about 200-400 psi/in/in. Preferably, no adhesive is used to secure ring 55 in the seal gland. Alternatively, secondary seal ring 55 may be attached or secured by adhesive to axial seal gland surface 39 (or last-machined surface 43) and to rigid seal ring 49 to enhance its sealing ability. Because secondary seal ring 55 remains stationary with last-machined surface 47 and does not seal relative rotary motion, it is a static seal, as opposed to seal 37, which is a dynamic seal.
For an 8½ inch bit, secondary seal ring 55 has an outer diameter D of approximately 2.480 inch and a radial width W is of about 0.211 inch. Outer diameter D is selected to be about 0.040 to 0.060 inch larger than the outer diameter of rigid ring 49. The inner surface or diameter and end 57 of secondary seal ring 55 are configured to be similar to and respectively conform to radial surface 45 and axial surface 39 of the seal gland. A radius R1 of about 0.085 inch and a tip radius R2 of about 0.015 inch are provided at the inner end of secondary seal ring 55.
Ring 55 also includes two raised ribs 57 which are approximately 0.025 inch to 0.030 inch wide and 0.010 inch to 0.014 inch high. The purpose of the ribs is to form high-stress areas to deter the entry of fluid and/or debris into the seal gland when secondary seal ring 55 is forced into contact with surface 39.
Ring 55 has an axial thickness t of about 0.095 inch (in the uncompressed or relaxed state), which is greater than the gap formed between axial surface 39 and the end of seal ring 49. The intent is to provide sufficient “squeeze” on secondary seal ring 55 between axial surface 39 and seal ring 49. In the preferred embodiment, this squeeze is approximately 20% to 25% of the uncompressed or relaxed radial thickness t of ring 55 using nominal values and with the cutter forced outward on the bearing shaft. A radius R3 of about 0.125 inch is provided to permit deformation of energizer ring 51 and to closely conform to it. The remaining width w of ring 55 is about 0.104 inch.
In the assembled configuration, the area in the seal gland bounded by surfaces 39 and 45, including rings 49, 51, and 55, is intended to be assembled so as to minimize or exclude air. Upon assembly, a continuous ring of heavy mineral oil is applied to at least axial surface 39, then secondary seal ring 55 is placed in the seal gland and energizer 51 and seal ring 49 are installed. This assembly process helps to insure that void areas are minimized and/or eliminated in the aforementioned area of the seal gland. In a later improvement shown in U.S. Pat. No. 7,413,037 the mineral oil was not needed as the shape of the backup ring was changed to have protrusions to fill the gaps that formerly were filled with the heavy mineral oil.
The problem with this design in the past is the tearing or breaking off of segments from the outer end of the backup ring 55 on the exposed face opposite surface 47 due to grit in the mud permeating toward this exposed surface that ultimately lead to seal failure of seal 37. The present invention addresses this issue in a variety of options. In one sense the material of the backup ring of the present invention is made harder but at the same time maintaining flexibility to address conflicting requirements for durability from well fluids and the need for application of a desired contact force between relatively moving surfaces 53 and 41 and a needed sealing force into the backup ring 55 into surface 39. Some of the ways this accomplished is material removal between opposed ends at the exposed edge where the removed portion is in the shape of a U or a V alone or in conjunction with support in the removed location that acts akin to a spring. Another option is to strengthen all or parts of the exposed edge with electron beam radiation to increase crosslink density at the extremities while leaving interior segments unaffected for control of the sealing force on the backup ring 55 and the contact pressure against relatively rotating surfaces 53 and 41.
These and other features of the present invention will be more readily apparent to those skilled in the art from a review of the detailed description of the preferred embodiment and the associated drawings while recognizing that the full scope of the invention is to be found in the appended claims.